Safety device for lifting electromagnets



Oct. 29, 1929. Y J, G 1,733,714

SAFETY DEVICE FOR LIFTING ELECTROMAGNETS Filed Mai'c'h 11, 1924 Patented Oct. 29, 192 9 UNITED STATES PATENT OFFICE JULIUS BING, or EIsENAcH, GERMANY, ASSIGNOR TO THE FIRM, MAGNET-WEEK G. M. B. H, EISENACH SPEZIALFABRIK run ELEKTROMAGNET-APPARATE, or

EISENACH, THURINGIA, GERMANY SAFETY DEVICE FOR LIFTING ELECTROMAGNETS Application filed March 11, 1924, Seria1 No. 698,524, and in Germany May 15, 1922.

My invention relates to improvements in safety devices for lifting electromagnets, and more particularly in safety devices designed to protect the coils of the electromagnets as against excessive heating.

As is known to those skilled in the art lifting electromagnets are in a high degree subject to destruction of the coils by excessive heating, because comparatively strong currents are supplied to the coils in order to produce a high lifting capacity with a low weight of the electromagnet. Therefore the coils and the insulation thereof are frequently destroyed by excessive heating.

It has heretofore been proposed to provide safety devices controlled by the temperature of the coils by switching out or short circuiting the coils, In apparatus of this type such as are now in use special leads are provided for connecting the electromagnet with the safety device, or each electromagnet is provided with a safety device of its own, and it has been necessary to provide a new safety device whenever one of them has been operative. are objectionable for the reason that there is a great danger of the load falling off the apparatus, because the safety device is liable to become operative at any time by accident.

The object of the improvements is to provide a system in which these objections are obviated, and with this object in view I provide a system which depends on the reduction of the current intensity caused by the increase of the temperature of the coil, the system preferably including a differential relay controlling a circuit break device and having two coils acting in oppositionto each other and one connected in series with the coil of the electromagnet and traversed by the current supplied thereto, and the other one connected in shunt to the coil of the electromagnet and receiving the same voltage. The power of the said 'coils is such that the coils compensate each other under normal conditions so that the circuit break device is not operated, and it is operated only if by the heating of the electromagnet the 'currentenergizing the coil connected in series with the electromagnet is weakened, so that the field of the coil con- Further, the systems now in use nected in shunt therewith exceeds that of the field of the series coil and the circuit break device is operated when the electromagnet has been heated to a predetermined temperature.

Other objects of the invention will appear from the following description of a preferred embodiment connected with a lifting electromagnet.

- The drawing shows my improved system in a diagrammatical way.

The lifting electromagnet is represented by the coil 1 thereof, the other parts being omit ted. As shown the said coil is connected by leads 2 and 3 with the net 4. The lead 2 includes a circuit break device 5 the terminals of which are adapted to be bridged by an arm of a rod 6 of the core of a solenoid 7. The coil 8 of the said solenoid is included in a subsidiary circuit branched from the conductors of the net 4:. One end of the coil 8 is directly connected by a lead 9 with one of the conductors of the net 4, and the other end of the said coil is connected by a lead 10 to a contact 11 of a pair of contacts 11, 12. The contact 12 is connected by a lead 32 with the other conductor of the net V The numeral 20 indicates a magnetic device which as shown is in the form of a differential relay, and which is included in the circuit of the lifting electromagnet, together with contacts 11 and 12 and other parts of the safety device. To illustrate this in a diagrammatical way the figure shows a base plate 31 on which the said parts are mounted, and which is located near the switch 36. The differential relay consists of a horseshoe magnet 33 the yoke of which carries two coils 19 and 20. The coil 20 is connected in series with the main coil 1 of the lifting electro magnetand it is included in the circuit 2, 3 supplying electric current to the lifting elec tromagnet. The coil 21 is connected in shunt to the coil 1. In the example shown in the drawing it is directly connected to the wires of the net 4. The armature 22 of the difi'erential relay 20 is I-shaped in cross-section and it is mounted on an axis 34 and connected with an arm 23 carrying a contact piece 35 at its end. To the base plate 31 a pin 24 is secured which provides a stop for limiting the outward movement of the arm 23, the latter being acted upon by a spring 26 attached with one end to the armature 22 and with the opposite end to a screw-threaded pin 27 adjustably mounted in a bore of a bracket 28 rising from the base plate 31. As shown nuts 29 and are screwed to the pin 27 at oppo- Site sides of the bracket 28, and by screwing the nuts in one or the other direction the pin 27 is shifted towards and away from the armature 22 for adjusting the tension of the spring 26.

In the path of the movement of the contact piece of the arm 23 two contacts 13 and 14 are provided which form'parts of a circuit 15, 16 including a source of electric energy 17 and an annunciator 18 which as shown consists of an electric bell.

The relative arrangement, the electric conditions, the number of the turns, the resistance, and other values of the coils 19 and 21 of the differential relay are such that the oppositely acting coils 19 and 21 compensate each other when the main coil 1 is cold and current of normal intensity is supplied thereto, so that no power is transmitted to the arm 23 in the direction of the arrow and the said arm is held by the spring 26 in contact with the stop 24:. If, however, the temperature of the electromagnet is unduly raised by continued or frequent use or by other reasons, the intensity of the current flowing through the circuit of the electromagnet and the coil 19 is reduced, said intensity depending on the voltage of the net (which is constant) and the resistance of the coil 1 (which depends on the temperature). Therefore the magnetic force of the coil 21 which is independent of the temperature of the coil 1 prevails over that of the coil 19, so that the armature 22 is turned against the action of the spring 26 and in the direction of the arrow a2. As the temperature of the coil 1 rises the arm 23 is further turned in the direction of the arrow 00, and it makes contact at first with the terminals 13, 1 1 and thereafter with the terminals 11, 12. By bridging the terminals 13, 1 1 the bell 18 is rung which calls the attention of the engineer to the fact that the coil 1 has been heated to a certain degree and that it is necessary to set off the load and to switch out the electromagnet to permit the same to cool off. Should this not be done the arm 23 proceeds and bridges the contacts 11, 12 and the circuit 19, 32 is closed. Thereby the core of the solenoid 7 is attracted and the main circuit 2, 3 is interrupted at 5, so that the electromagnet 1 is deenergized and further heating is prevented.

Preferably the coils 19 and 21 are made from a material of low temperature coeflicient in order that the ampere turns of the coil 19 be practically constant at any temperature. By the varying tension of the spring the intensity of the current causing interruption of the circuit of the coil 1 may be varied. It will be understood that the invention is not limited to the use of the solenoid 7 and that the circuit of the coil 1 may also be broken directly by the arm 23 of the differential relay, or other means may be provided in lieu of the said relay operating by differential action of two magnet coils one connected in series with the main coil to be protected and the other one connected in shunt.

lVhile in describing the invention reference has been made to an example in which the safety device is said to be mounted on the lifting electromagnet, I wish it to be understood that this construction has been mentioned only by way of example and that in some cases it will be preferable to mount the said device independently of the lifting electromagnet, so that it is not exposed to'the same rough handling as the electromagnet. A valuable feature of my improved system is that no additional leads are needed for the apparatus to be protected, and that no elements need be exchanged or repaired after the device has operated. Therefore, my improved system is always operative. Further, the lifting electromagnet may at any time be exchanged and replaced by another one, and

the safety device can be used in connection with lifting electromagnets of different sizes and current intensities, by regulating the circuit breaking device by suitable means such as the spring 26 or resistances connected in series or in shunt with the coils of the relays. In order that this may be more readily done the device is preferably mounted at a readily accessible place such for example as the engineers cage of a crane.

I claim:

1. In an electro-magnetic apparatus of the class described the combination of main circuit for normally operating said apparatus by working current, a safety device for interrupting said main circuit, electro niagnetic means for actuating said safety device, said electro-magnetic means comprising two differentially wound coils one in series with and the other in shunt to said main circuit, whereby said safety device will be caused to respond and interrupt said main circuit by a reduction of the current strength therein, said reduction being due to an increase of re sistance in said electromagnetic apparatus caused by the heat developed therein by an excessive current in said main circuit.

2. In an electro-magnetic apparatus of the class described, the combination ,of a main electromagnet Winding, a main circuit including said winding, :1. contact make-andbreak device included in said main circuit, electrical means for operating said contact make-and-break device, a further circuit including said electrical means and being provided with terminals and a switch for closlit) lli

ing the same, a diiierential magnet having an armature operatively connected with said switch and a pair of electrically oppositely acting coils one in series with and the other in shunt to said main electro-magnet winding, said differential magnet tending to move said armature and therewith said switch into circuit closing position during an increase of the resistance in the electromagnetic apparatus and a corresponding weakening of the current within said series coil. 1

3. In an electro-magnetic apparatus of the class described, the combination of a main electromagnet winding, a main circuit including said winding, a contact make-andbreak device in said main circuit, electrical means for operating said contact make-andbreak device, a circuit including said electrical means and having terminals and a switch for closing the same, a differential magnet having an armature operatively connected with said switch and a pair of electrically oppositely acting coils one in series with and the other in shunt to said main electromagnet winding, said differential magnet tending to move said switch into circuit closing position during an increase of the re sistance in the electromagnetic apparatus and a corresponding weakening of the current within said series coil, a spring tending to keep said switch out of circuit closing position, and means for varying the tension of said spring.

4. In an electro-magnetic apparatus of the class described, the combination of a main circuit for normally operating said apparatus by working current, means for interrupting the said main circuit, means comprising two diflerentially wound coils adapted to actuate said interrupting means through their resultant flux only, one of said coils being in series with the said main circuit to provide for a reduction of the current flowing through saidcoil and correspondingly for an effective increase of said resultant flux due to an increase of resistance in said electromagnetic apparatus caused by the heat developed therein by an excessive current in said main circuit, the other one of said coils being in shunt with said main circuit.

In testimony whereof I hereunto aifix my signature.

JULIUS BING. 

